1. Field of the Invention
The present invention relates to a method and apparatus for injection of chemicals, particularly ammonia-based reagents and hydrocarbons, into the convective pass of a coal, oil or gas fired furnace/boiler to reduce pollutants, such as NOx and mercury, in the furnace/boiler flue gases.
2. Brief Description of the Related Art
Power generator exhaust or flue gases present a major source of air pollutants, such as nitrogen and sulfur oxides and mercury which contribute to a number of environmental problems. Photochemical smog is formed when NO2 undergoes a series of reactions in the presence of sunlight and airborne hydrocarbons. The consequences of photochemical smog are harm to human health, degradation of many materials, and reduced yield of crops in agriculture. NOx also reacts with the water in the atmosphere to form nitrous and nitric acids. Acid rain has been associated with the deterioration of buildings and statues made of granite, limestone and marble. Another consequence of acid rain is its effect on aquatic life. Typical pH levels in lakes and streams located in regions liable to acid rain have dropped from a neutral 7 to a weakly acidic 4.
NOx is formed during combustion processes. There are two mechanisms for the formation of NOx. “Fuel” NOx is a result of the oxidation of the nitrogen chemically bound in the fuel during the burning process. “Thermal” NOx is formed in the high temperature combustion zone of a boiler when the molecular species of nitrogen and oxygen within the combustion air react to form NOx.
Combustion modification technologies, such as low NOx burners, over-fire air and staged combustion, all limit the formation of “fuel” NOx by reducing the amount of oxygen present during the period in which the nitrogen species are being evolved from the fuel matrix. By reducing the overall within the combustion zone the “Thermal” NOx can also be reduced.
The technologies mentioned above are all capable of achieving upwards of 50% NOx reduction; however, tightening pollution control legislation requires upwards of 80% reduction. It is, hence, now necessary for coal, gas and oil fired boilers to utilize some sort of effluent gas cleanup system.
Gas clean-up technology to date can be classified into two main groups: selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). NOx is reduced to water and nitrogen in both processes. The most common form of SCR consists of a catalytic reactor positioned downstream of the economizer. A NOx reducing chemical, such as ammonia or urea, is injected just prior to the reactor. SNCR directly injects the reducing chemical into the furnace or high temperature convective pass of the boiler itself. Due to the high temperature within the furnace, reasonable NOx reductions can be achieved without contact with a catalyst. SNCR injection is achieved by use of wall-type injectors or with retractable lances based on soot blower technology. Ultimately SCR achieves much higher NOx reduction than SNCR; however, SNCR has advantages over SCR that include cheaper installation cost and lower power consumption.
Mitsui Babcock has marketed a process that reduces pollutants, such as NOx and mercury, (hereinafter referred to as the “autocatalytic process”) as an alternative to SCR and SNCR. Autocatalytic processes achieve the high NOx reduction associated with SCR technology but in combination with the low installation and power consumption costs associated with SNCR. Autocatalytic processes remove NOx in flue gas containing 2 to 18% O2 average concentration, noting that locally O2 can be much lower in a large boiler. The NOx reduction is achieved by introducing a controlled amount of hydrocarbon to “auto-catalyze” the NOx reduction process and to release heat auto-thermally for self-sustaining “autocatalytic” reactions. Reductants, such as ammonia-based reagents, are introduced as reactants and, in conjunction with the hydrocarbon, reduce NOx concentration in the flue gas. The plasma of the hydrocarbon creates the “autocatalytic” conditions.
The hydrocarbon is introduced into the flue gas using a carrier gas, such as steam. The preferred hydrocarbon is natural gas although other hydrocarbons, such as propane, have been used and found to be suitable. The amount of hydrocarbon injected into the furnace is a function of the temperature in the injection zone and will generally be 0-2% of furnace/boiler heat input. In a typical SNCR process, high temperatures are required to produce radicals that are required to form the NH2 radicals from ammonia for the NOx reduction reaction. In autocatalytic processes, the auto-ignition of the hydrocarbon provides a source of these radials, and the NOx reduction reaction can occur at lower temperatures and with higher NOx reductions than normally expected with SNCR. For further details of an autocatalytic process, reference is made to U.S. Pat. No. 5,985,222, No. 6,066,303 and No. 6,348,178 to Sudduth et al which are incorporated herein by reference.
Autocatalytic processes have a number of benefits over traditional SCR and SNCR technologies. They have a lower capital cost compared with SCR technology but still achieve similarly high NOx reductions, which are much higher than the reductions associated with SNCR technology. They also avoid the problems associated with the SCR solid catalyst which include chemical poisoning, physical plugging, sintering from temperature excursions, increased pressure drop resulting in the need for new, larger induced draft fans and disposal of a hazardous waste.
U.S. Pat. No. 4,115,515 to Tenner et al, U.S. Pat. No. 4,782,771 to Bergkvist, U.S. Pat. No. 4,950,573 to Flockenhaus et al, U.S. Pat. No. 5,240,689 to Jones, U.S. Pat. No. 5,326,536 to Carter, U.S. Pat. No. 5,645,690 to Viel Lamore et al, U.S. Pat. No. 5,489,419 to Diep et al, U.S. Pat. No. 5,489,420 to Diep, U.S. Pat. No. 5,536,482 to Diep et al, U.S. Pat. No. 5,685,243 to Gohara et al, U.S. Pat. No. 5,820,838 to Tsuo et al, U.S. Pat. No. 6,190,628 B1 to Carter and U.S. Pat. No. 6,453,830 B1 to Zauderer and U.S. Patent Application Publication No. US 2002/0025285 A1 to Comparato et al are representative of the injection of chemicals into flue gases on a zonal or areal basis; however, all have the disadvantages of having injection of a single concentration of chemicals along the length of an elongate lance passing through a flue gas pass and/or having nozzles mounted on the walls of the flue pass such that chemicals cannot be precisely injected in interior zones of the flue gas pass in an efficient and cost effective manner.